found this in a magazine. thought it might be interesting:
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Air Conditioner Retrofit Update
Paul Weissler - Home Mechanix, September 1994

At least one major manufacturer of ozone-depleting R-12 refrigerants
will continue production right up to the December 31, 1995 phase-out
deadline.  This has allowed carmakers and A/C shops to stockpile R-12.
But even though supplies remain plentiful, the cost of an R-12 recharge
has risen.  It's now $125-150.  Meanwhile, price estimates for a basic
R-134a retrofit have dropped to about the same level.

We'll show why switching to R-134a can make financial sense when your
R-12 A/C systems springs a leak.  We'll also detail the parts involved
for all the major brands, discuss which systems are still expensive to
switch ... and how much you'll pay.

Warranty Woes
Since most U.S. and import carmakers still have enough R-12 to meet
warranty needs, most will simply fix existing systems without
switching them to R-134a.  A notable exception is Volvo.  The Swedish
automaker has destroyed its leftover R-12 and offers retrofits for all
of its R-12 vehicles.  As a training program for dealers, GM will be
offering owners of 1991 to 1993 Buick LeSabres and Olds 88s under
warranty a choice between an R-12 repair or a switch to R-134a.

Still, most car owners who want or need a retrofit will probably have
to pay for it.  Some may weigh the costs and decide to stick with R-12,
especially if a recharge comes under warranty.  Unfortunately, R-12
systems could leak again later on when the cost of an R-12 fill-up will
probably be considerably higher.

Another retrofit incentive: While recharge cans of R-12 are no longer
available to consumers, you can buy R-134a, along with kits that allow
you to recharge your own system.

Low-Cost Conversions
Long-term testing has shown that R-134a swaps aren't as involved as
industry experts had believed.  Only repaired hoses with barb fittings
and worm-drive clamps must be replaced.  And most refrigeration-line
fitting seals can stay.

Many existing compressors and compressor-shaft seals were thought to
be victims of the refrigerant shift over.  Nippondenso claimed the
shaft seals on almost all of its compressors would leak too much with
R-134a.  But their tests proved too severe.  Now it seems only a few
compressors will need replacing.  The major ones include:
* The Techumseh HR-980 used on many Ford vehicles through 1989.
* The Panasonic/Matsushita compressor used on many Mazdas and the
    Mazda-engineered Ford Probe through 1993.
* The Seiko-Seiki compressors used on some Japanese and European cars.
* The Sanden compressors used on five-cylinder Volvos.

Correcting Compressors
Problem compressors use seals made of Viton, a synthetic rubber that
isn't compatible with R-134a.  Check the label attached to your car's
compressor to see if it's on the list.  If it is, here are the fixes:
* Replace the Tecumseh unit with a Ford FS-6 compressor.  You can get
    the adapter kit from your dealer.
* Swap the Panasonic/Matsushita compressor for a 1994 model, which is
    R-134a compatible.
* Switch the Seiko-Seiki unit for an R-134a model from 1993 or later.
* The Sanden compressors in Volvos only use Viton in the shaft seal,
    and replacing that part is a routine job for an A/C specialist.
    Cost is $110 to $175.

Even if your car has a problem compressor that isn't covered above, or
the recommended fix is too expensive, you still have options.  A/C
shops can use a compressor conversion kit and adapt a rebuilt R-12
Sanden or Zexel unit for your car.  In some cases, they can even install
an R-134a Sanden compressor.  These swaps average about $350.

Be sure the technician checks your car's A/C system with an electronic
leak detector that meets the new standards set by the Society of
Automotive Engineers.  Technicians whould also use a high-performance
vacuum pump to ensure that no traces of R-12 are left in the system.
Service shops that belong to the Mobile Air Conditioning Society
(215 541-4500) are your best bet.

Climate Concerns
R-134a has smaller molecules than R-12 and operates at higher pressures.
As part of a retrofit, have the shop install a high-pressure cutout
switch that disengages the compressor if system pressures get too high.
You may temporarily lose A/C when driving in heavy traffic on hot days,
but it's probably worth it since adding a fan or high-performance
condenser can get expensive.  Front-wheel drive cars may not need the
installation since many already have the switch.

Oil Options
The mineral oil that lubricates R-12 A/C systems doesn't mix with
R-134a's synthetic oil, so retrofits include a new synthetic oil.  A/C
technicians choose between two categories:  poly alkylene glycol (PAG)
and polyol esters.

All new car systems use some form of PAG.  Most car makers' retrofit
programs specify a particular PAG, though Volvo uses an ester.  You
can avoid a mix-up by making sure the technician attaches a label to
a retrofitted system stating which oil and refrigerant it contains.
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For More Information, Contact:

Castrol Industries, Inc.     (215) 433-5220
775 Louis Dr.                <sidebar shows Castrol Icematic Retro100
Dept. HM994                   oil, seals, new valves, and sticker in
Warminster, PA 18974          a kit>

Mobile Air Conditioning Society (MACS)   (215) 541-4500
Box 97
Dept. HM994
E. Greenville, PA 18041
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Which System Needs What*

Vehicle/System #         Parts Needed##          Price (includes Labor)
........................ Chrysler ................................
1988-93 all except below Receive-dryer~, high    $150-190 plus $35 for
                         pressure switch if      high-pressure cutout
                         needed                  switch
1988-92 Eagle Premier,   Accumulator, high-      $225-275
  Dodge Monaco           pressure cutout
                         switch
........................ Ford ....................................
1988-93 with orifice-    Accumulator, high-      $180-220 plus $35 for
  tube system and Ford   pressure cutout         high-pressure cutout
  or Nippondenso         switch if needed        switch
  compressor
1988-93 with Panasonic/  Same as above plus      $950-$1000@
  Matsushita R-12        R-134a Panasonic/
  compressor             Matsushita compressor
1985-89 with Tecumseh    Accumulator, high-      $850-900@
  compressor             pressure cutout switch,
                         FS-6 compressor
                         conversion kit
........................ General Motors ..........................
1988-93 all except       High-pressure cutout    $90-130 plus $35 for
Saturn                   switch on rear-drive    high-pressure cutout
                         models                  switch
1990-93 Saturn           No additional parts     $90
                         necessary
........................ Mazda ...................................
1988-93                  Receive-dryer~, high-   $150-190 plus $35 for
                         pressure cutout switch  high-pressure cutout
                         if needed               switch
1988-93 with Panasonic/  Accumulator, high-      $950-1000
  Matsushita R-12        pressure cutout switch
  compressor             on cars not so equipped
                         and R-134a Panasonic/
                         Matsushita compressor
........................ Mitsubishi ..................................
1988-93                  Receive-dryer^, high-   $150-190 plus $35 for
                         pressure cutout switch  high-pressure cutout
                         if needed               switch
........................ Nissan ......................................
1988-93                  Receive-dryer^, high-   $150-190 plus $35 for
                         pressure cutout switch  high-pressure cutout
                         if needed               switch
........................ Subaru ......................................
1988-93                  Receive-dryer^, high-   $150-190 plus $35 for
                         pressure cutout switch  high-pressure cutout
                         if needed               switch
........................ Toyota ......................................
1988-93                  Receive-dryer^, high-   $150-190 plus $35 for
                         pressure cutout switch  high-pressure cutout
                         if needed               switch
........................ Volvo .......................................
1985-91 with Sanden      Volvo retrofit kit with $275-325
  compressor             accumulator, new shaft
                         seal for compressor
1985-91 with             Volvo retrofit kit      $150
  Nippondenso compressor

* - These figures are estimates based on interviews with A/C parts
    suppliers and technicians.  Information reflects parts available
    and modesl covered by the aftermarket industry as of this writing.
    Parts and procedures may change and additional models will be
    covered as new parts and information become available.
# - Determine which compressor is on your vehicle by checking the label
    attached to it.
##- In addition to standard retrofit kit parts, which include fittings,
    compressor label, O-rings and lubricant.
@ - $485-600 if A/C shop adapts a Sanden or Zexel compressor.
~ - Deduct $60 for cars less than three years old, since they don't
    need a receive-dryer.